I found an article on Wikipedia that says that the biggest problems with the McKeen cars was reliability of the engines. I think that they had too many moving parts. Here is the article: https://en.wikipedia.org/wiki/McKeen...ny#Motor_truck
The biggest problem they were having was trying to adapt marine engines for rail use. Also the lack of a clutch hampered operation. Internal combustion electric drive was in it infancy, and used successfully by other makers. The Denver and Rio Grande railroads Galloping Goose used a Pierce Arrow automotive power plant with success. The problem with the McKeen cars was an unreliable power plant.

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...the horizontal cast steel engine bed was also described as a structural member. The straight-6 engine had an 11-inch (279 mm) bore and 15-inch (381 mm) stroke, for a total displacement of 8,553 cubic inches (140.16 L); it developed 300 horsepower (220 kW).

William McKeen was the Union Pacific Railroad’s Superintendent of Motive Power when he, and/or staff, came up with this design. The Railroad built the first four cars, and subsequent units were constructed by McKeen in leased space at Union Pacific’s Omaha Shops. So McKeen started a company to build this design.

I wonder if that was Union Pacific didn't want to risk liability?
Couldn't convince stockholders this was a geed investment?
Up's charter wouldn't allow to selling to other railroads?
McKeen's name was on the patents, unlikely if he was working for UP?
It was a sweetheart deal between McKeen and the UP brass?

I also wonder what engines were used in the first four?
FOUND IT!

Quote:

The first McKeen Car was built in 1904-5, and was reported complete in March of 1905. This car was built with a steel frame and wood body, was 31′ feet long, and sat 25 passengers. It had a Riotti 50 H.P. Gasoline engine, which, when tested in the Omaha Yards, proved too weak for a train car of that size, and in two months, a 100 H.P. Riotti engine was fitted before it was sent off to Grand Island, Neb. for further testing.

They whole concept of an unassailable giant like Union Pacific, trying to carry the public on the cheap, is kind of scary. I mean if you try to raise corn or rabbits and fail, then it's too bad. Carrying 80 people and fail, is much more serious. But in those days, Union Pacific might refund the price of the ticket... to the next of kin. Think how little White Star paid to Titanic victims.

OK, after the first four cars built by UP, they started using an engine of McKeen's design, except 3 Rottis, 1 Samet, and 1 Sterling.
The cars McKeen built for other railroads had his engine or Hall-Scott, or Winton engines.

The Shay locomotive was the most widely used geared steam locomotive. The locomotives were built to the patents of Ephraim Shay, who has been credited with the popularization of the concept of a geared steam locomotive. Although the design of Ephraim Shay's early locomotives differed from later ones, there is a clear line of development that joins all Shays.
The strength of these engines is that all wheels, including, in some engines, those under the tender, are driven so that all the weight develops tractive effort. A high ratio of piston strokes to wheel revolutions allowed them to run at partial slip, where a conventional rod engine would spin its drive wheels and burn rails, losing all traction.